Make ball move automatically

So my problem is I need to make the ball move around the screen automatically. So when it reached one of the screen it needs to go the other way. If I could also get help on how I would use trig if the ball hits a corner to make it bounce a certain way that would be amazing too as I am going to do that later!

At the moment my ball will just keep travelling right.
And yes I have a variable for my screen width and screen height as well as for my ball width and ball height.